Multiphase polymer blends are of major economic importance in the polymer industry. Some examples of the multiphase polymer blends involve the impact modification of thermoplastics by the dispersion of rubber modifiers into the thermoplastic matrixes. In general, commercial polymer blends consist of two or more polymers combined with small amounts of a compatibilizer or an interfacial agent. Generally, the compatibilizers or interfacial agents are block or graft copolymers which can promote the forming of small rubber domains in the polymer blends so as to improve their impact strength.
In many applications, blends of polypropylene (PP) and ethylene/α-olefin copolymers are used. The ethylene/α-olefin copolymer functions as a rubber modifier in the blends and provides toughness and good impact strength. In general, the impact efficiency of the ethylene/α-olefin copolymer may be a function of a) the glass transition (Tg) of the rubber modifier, b) the adhesion of the rubber modifier to the polypropylene interface, and c) the difference in the viscosities of the rubber modifier and polypropylene. The Tg of the rubber modifier can be improved by various methods such as decreasing the crystallinity of the α-olefin component. Similarly, the viscosity difference of the rubber modifier and polypropylene can be optimized by various techniques such as adjusting the molecular weight and molecular weight distribution of the rubber modifier. For ethylene/higher alpha-olefin (HAO) copolymers, the interfacial adhesion of the copolymer can be increased by increasing the amount of the HAO. However, when the amount of the HAO is greater than 55 mole % in the ethylene/HAO copolymer, the polypropylene become miscible with the ethylene/HAO copolymer and they form a single phase and there are no small rubber domains. Therefore, the ethylene/HAO copolymer with greater than 55 mole % of HAO has a limited utility as an impact modifier.
For thermoplastic vulcanizates (TPV's) where the rubber domains are crosslinked, it is desirable to improve properties such as compression set and tensile strength. These desirable properties can be improved by decreasing the average rubber particle size. During the dynamic vulcanization step of TPV's comprising polypropylene and a polyolefin interpolymer such as ethylene/alpha-olefin/diene terpolymers (e.g., ethylene/propylene/diene terpolymer (EPDM)), there must be a balance of compatibility of the terpolymer with the polypropylene. In general, EPDM has a good compatibility with polypropylene, but the compatibility can only be marginally improved with increasing propylene level in EPDM.
Despite the availability of a variety of polymer blends, there is a need to continue to develop polymer blends with improved properties.